Programmed cell death protein-1 (PD-1) is a type I membrane protein of 268 amino acids and is a member of the extended CD28/CTLA-4 family of T cell regulators PD-1 (The EMBO Journal (1992), vol. 11, issue 11, p. 3887-3895). Human PD-1 cDNA is composed of the base sequence shown in EMBL/GenBank Acc. No. NM_005018 and mouse PD-1 cDNA is composed of the base sequence shown in Acc. No. NM_008798, and those expressions are observed when thymus cells differentiate from CD4−CD8− cell into CD4+CD8+ cell (International Immunology (1996), vol. 18, issue 5, p. 773-780., J. Experimental Med. (2000), vol. 191, issue 5, p. 891-898). It is reported that PD-1 expression in periphery is observed in myeloid cells including T cells or B lymphocytes activated by stimulation from antigen receptors, or activated macrophages (International Immunology (1996), vol. 18, issue 5, p. 765-772).
PD-1 is a member of the CD28 family of receptors, which includes CD28, CTLA-4, ICOS, PD-1, and BTLA. The initial member of the family, CD28, was discovered by functional effect on augmenting T cell proliferation following the addition of monoclonal antibodies (Hutloff et al. (1999) Nature 397:263-266; Hansen et al. (1980) Immunogenics 10:247-260). Two cell surface glycoprotein ligands for PD-1 have been identified, PD-1 and PDL-2, and have been shown to down-regulate T cell activation and cytokine secretion occur upon binding to PD-1 (Freeman et al. (2000) J. Exp. Med. 192:1027-34; Latchman et al. (2001) Nat. Immunol. 2:261-8; Carter et al. (2002) Eur. J. Immunol. 32:634-43; Ohigashi et al. (2005) Clin. Cancer Res. 11:2947-53). Both PD-1 (B7-H1) and PD-L2 (B7-DC) are B7 homologs that bind to PD-1. Expression of PD-1 on the cell surface has also been shown to be upregulated through IFN-γ stimulation.